Protect WebURLLoaderImpl::Context while receiving responses.

      A client's didReceiveResponse can cancel a request; by protecting the
Context we avoid a use after free in this case.

Interestingly, we really had very good warning about this problem, see 
https://codereview.chromium.org/11900002/ back in January.

R=darin
BUG=241139

Review URL: https://chromiumcodereview.appspot.com/15738007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202821 0039d316-1c4b-4281-b951-d872f2087c98